1. Technical Field
The present invention relates to a memory device and a method for fabricating the same, and more particularly, to a memory device in which data is written or read by a switching operation of a bit line that is inserted into a trench formed between a plurality of word lines, and a method for fabricating the same.
2. Discussion of Related Art
In general, memory devices for storing data can be classified as volatile memory devices and nonvolatile memory devices. Among memory devices, a volatile memory device, such as a dynamic random access memory (DRAM) or a static random access memory (SRAM), has a relatively high data input/output speed, but loses stored data when the power supply is removed. A nonvolatile memory semiconductor device, such as an erasable programmable read only memory (EPROM) or an electrically erasable programmable read only memory (EEPROM) has a relatively low data input/output speed, but retains data even when the power supply is removed.
Meanwhile, such a conventional memory device commonly includes a metal oxide semiconductor field effect transistor (MOSFET) based on metal oxide semiconductor (MOS) technology. For example, stacked gate type transistor memory devices stacked on a silicon semiconductor substrate and trench gate type transistor memory devices buried in the semiconductor substrate are under development. However, the MOSFET requires a channel having a suitable width and length over a certain substrate area to prevent a short channel effect, and requires that a gate insulating layer formed between a gate electrode on the channel and the semiconductor substrate has a significantly small thickness. Thus, it is difficult to implement a MOSFET-based memory device of a nano-scale microstructure.
For this reason, memory devices for substituting for the MOSFET are being actively studied. In contemporary semiconductor technology, micro electromechanical system (MEMS) and a nano electromechanical system (NEMS) are being developed and used. Among them, a memory device having a carbon nanotube structure is disclosed in U.S. Pat. No. 6,924,538 entitled “Devices Having Vertically-disposed Nanofabric Articles and Methods of Making,” incorporated herein by reference.
A conventional memory device in accordance with the above carbon nanotube structure will now be described with reference to FIGS. 1 and 2. FIGS. 1 and 2 are cross-sectional views illustrating a conventional memory device. Referring to FIGS. 1 and 2, a conventional memory device includes a plurality of electrodes 304 and 306 having a predetermined channel or gap interposed therebetween, and a nanotube piece 308 vertically passing through the channel or gap at a spacing from the plurality of electrodes 304 and 306 and, and storing data as the nanotube piece 308 is brought into contact with one of the plurality of electrodes 304 and 306 or separated from the electrodes.
The plurality of electrodes 304 and 306 are formed to be symmetrical to each other with respect to the nanotube piece 308, the ends of which are anchored at a center of the channel or gap in a horizontal direction. For example, the plurality of electrodes 304 and 306 are formed of a conductive metal or a semiconductor material. Insulating support structures support the plurality of electrodes 304 and 306, while insulating upper and lower portions of the nanotube piece 308 inserted into the channel or gap formed between the plurality of electrodes 304 and 306 from the electrodes 304, 306.
The nanotube piece 308 may vertically pass through the channel or gap formed between the plurality of electrodes 304 and 306 and be brought into contact with any one of the plurality of electrodes 304 and 306 under a predetermined condition. For example, the conventional memory device can store one-bit data corresponding to a structure 310 in which the nanotube piece 308 is bent toward and brought into contact with the first electrode 304 of the plurality of electrodes 304 and 306 to which a charge having an opposite polarity to charge applied to the nanotube piece 308 are applied and a structure 314 in which it is bent toward and brought into contact with the second electrode 306. However, standby power is required to sustain the electrical contact between the nanotube piece 308 and the plurality of electrodes 304 and 306. Accordingly, it is difficult to implement a nonvolatile memory device using this configuration.
A method for fabricating such a conventional memory device will now be described. First, the first electrode 304 is formed on an insulating substrate. A trench (not shown) exposing the insulating substrate at one side of the first electrode 304 is then formed.
A first sacrificial layer (not shown), the nanotube piece 308, and a second sacrificial layer (not shown) are stacked to a predetermined thickness on sidewalls of the trench, and a first insulating support (not shown) is formed to a predetermined thickness on the bottom of the trench.
Then, the first sacrificial layer and the second sacrificial layer may be removed and the second electrode 306 spaced apart from the nanotube piece 308 by a predetermined distance may be formed on the first insulating support inside the trench. However, it is difficult to from the second electrode 306 spaced apart by a predetermined distance from the nanotube piece 308, which is exposed to the sidewalls of the trench. For example, even though a mask layer for forming the second electrode 306 is formed on the trench, it is difficult to vertically form a conductive metal layer stacked at a certain interval relative to the nanotube piece 308 inside the trench. That is, it is difficult to form the second electrode 306 in a symmetrical structure, which is opposite to the first electrode 304 with predetermined channels or gaps formed at both sides of the nanotube piece 308.
Increase in the protruding distance of the mask layer from a top end of the sidewall of the trench leads to increase in the distance between the second electrode 306 and the nanotube piece 308, requiring a greater electrical attractive force for contact between the first electrode 304 or the second electrode 306 and the nanotube piece 308. Accordingly, a large amount of power can be consumed for writing data to the conventional memory device.
As described above, the conventional memory device described above and a method for fabricating the same have the following problems.
First, it is difficult to form the second electrode 306, which is formed inside the trench exposed by the first electrode 304 on the insulating substrate, to be symmetrical with respect to the nanotube formed at a predetermined distance from the sidewall of the trench. Thus, reliability and production yield are degraded.
Second, the distance between the second electrode 306 and the first electrode 304 at both sides of the nanotube increases in proportion to the distance by which the mask layer used for forming the second electrode 306 inside the trench protrudes from the sidewall of the trench, and consumption of power required for data writing increases due to increase in the bending distance of the nanotube. Thus, efficiency is degraded.
Third, charge must be continuously supplied to either the nanotube piece 308 or the plurality of electrodes 304 and 306 in order to maintain the contact between any one of the plurality of electrodes 304 and 306 and the nanotube piece 308. Accordingly, standby power consumption increases. When the charge supply is removed, information corresponding to the contact state of the nanotube piece 308 cannot be maintained. Thus, it is impossible to implement a nonvolatile memory device.